The present invention relates generally to inkjet inks and, more particularly, to pigment-based inks wherein the pigment is solubilized.
Inkjet inks comprise a combination of vehicle (one or more water-miscible organic co-solvents) and colorant (one or more water-insoluble dyes or water-soluble dyes or pigments), together with various additives (pH buffers, drop stabilizers, surfactants, algicides, and the like). While early work focused on water-soluble dyes, due to their brilliance, more recent work has centered on pigments, which are water-insoluble colorants, due to their superior waterfastness and smearfastness properties as compared to water-soluble dyes.
Solubilization of pigments in water remains a challenging problem. As colorants, pigments can afford many achievable attributes that heretofore dyes have not been able to achieve, i.e., waterfastness, lightfastness, and exceptionally high print quality. Non-covalent attachment of water-solubilizing polymers to pigments has been the most commonplace method for rendering them water-soluble. However, these polymers easily become displaced from the pigment particle over time and, consequently, a loss of dispersion stability results.
The best method for solubilizing pigments is direct covalent attachment of the solubilizing group. Cabot Corp. succeeded in employing this method via a diazotization reaction on carbon black and colored pigments; see, e.g., U.S. Pat. No. 5,554,739 to Belmont, U.S. Pat. No. 5,571,311 to Belmont et al, U.S. Pat. No. 5,922,118 to Johnson et al, PCT Application WO 96/18695, and PCT Application WO 96/18696. The diazotization reaction has some distinct advantages, but also suffers from some drawbacks, i.e., it must be run under acidic solution, the diazonium salt can only be made from aromatic amines, etc.
It is desirable to find other substitution reactions which accomplish the same task, but under different conditions and allow more flexibility in the synthesis of the self-dispersing pigment. Thus, specific chemistry on the surface of the pigment must be considered for the substitution reaction to occur.
In accordance with the present invention, a method for dispersing a pigment in an aqueous solution is provided. The method comprises:
(a) providing the pigment, the pigment distinguished in having an azo group and either a xcex2-diketo structure or a hydroxyl group, each conjugatively associated with the azo group;
(b) treating a surface of the pigment with a base having a sufficient strength to generate a negative charge on the pigment; and
(c) reacting the negatively charged pigment with either (1) a first hydrophilic molecule containing at least one functional group comprising a carbonxe2x80x94carbon double bond and a carbonyl that is alpha to the carbonxe2x80x94carbon double bond which reacts via addition to form a hydrophilic group on the pigment or (2) a second hydrophilic molecule containing at least one functional group selected from the group consisting of anionic functional groups, cationic functional groups, and zwitterionic functional groups and at least one suitable leaving group which reacts via substitution to form a hydrophilic group on the pigment.
The reaction in step (c) either comprises (1) a Michael addition reaction and exemplary hydrophilic molecules comprise a vinyl, an acrylate, a methacrylate, or a vinyl sulfone or (2) a nucleophilic substitution reaction and exemplary suitable leaving groups comprise chloride, sulfate, ethyl sulfate, bromide, tosylate, or methane sulfonate.
The treated pigment is rendered comparatively soluble in water-based inks, thereby eliminating the need for any dispersant to disperse the pigment.
Further in accordance with the present invention, an inkjet ink is provided, wherein the inkjet ink comprises a vehicle and a colorant, with the colorant comprising the treated pigment.